Great post, James.
I find there is a kind of spiritual, transcendent quality to the Celtic aesthetic in general; even something melancholy can be beautiful. Plus, the Ladies have a positivity and sincerity that's uncommon in entertainment today. And (as one of them described) the music is all about land, and love, and life, so there's a relatability there, too. So for me, it's who they are, combined with what they tap into.